Many electrical devices, particularly portable or remote electrical devices are powered by batteries. Batteries usually require recharging or replacing on a regular basis for continued operation of the electrical device. However, it is not always desirable or possible to connect a device to a mains power supply for recharging, or easy to gain access to the device for replacing the batteries. Moreover, it may not be possible to rely on a battery to perform with 100% reliability over extended periods and/or extreme operational/environmental conditions. As such, other means for powering/charging devices have been explored.
One proposal for obtaining electrical energy for electrical devices is by means of energy harvesting at the electrical device. Energy harvesting involves converting energy from clean, freely available sources into electrical energy and represents an environmentally friendly solution. Commonly used free energy solutions for powering electrical devices include solar, thermal and vibrational energy sources. For example, practical solar harvesters, such as solar panels and solar rectennas, utilise photovoltaic cells to convert photons into electrical energy via the photovoltaic effect. However, solutions such as solar harvesters are not appropriate for many types of remote electrical devices and are also not ideal for use in many geographical locations.
An alternative proposal is the wireless delivery of electrical power. For example, high power RF sources can be used to deliver RF power in a specific frequency band to recipient devices. The recipient devices then receive the RF energy and convert it into an electrical form, which can be used to power the device. Such systems can work efficiently in certain environments. However, such systems rely upon the presence of high energy RF transmitters, which reduces the overall attractiveness of such systems due to the need to install infrastructure, and therefore limits the number of possible applications that can use such technology.
Due to the expanse in the mobile telecommunications industry in recent decades and the corresponding increase in ambient RF energy, some research has looked at whether such ambient RF energy could be harvested. Presently, few practical energy collection solution for harvesting ambient RF energy have been reported, despite the abundance of such energy. One of the key challenges in respect of harvesting RF energy includes the collection of very low RF power, which can be as low as 10 nW. At present, RF energy harvesting does not look to be a viable option.
There is therefore still a requirement for alternative sources of power for remote devices.